1. Field of the Invention
The present invention relates to a degaussing system for erasing of various magnetic media generally used for the storage of information through the use of special winding details in a filar wound coil or coils which provide a rotating decaying magnetic field and allow for improved field uniformity and which provide matching electrical and magnetic parameters for optimal performance.
2. Description of the Prior Art
Various degaussing systems for erasing magnetic media are known in the art. For example, although not applied to magnetic media in bulk, U.S. Pat. No. 2,962,560 discloses a system which utilizes decaying AC magnetic field as an optimal degaussing process. The system in the '560 patent applies a damped sinusoidal field along an information-bearing track of a revolving magnetic drum, exposing each point on the track to a field that reverses and decays.
In more recent art, in which individual tracks on magnetic tapes are erased, several factors including the period of a sinusoidal AC field of constant amplitude; the weakening of that field with distance from its source, such as an erase head; and the motion of tape with respect to that source; cooperate to cause the decaying AC effect, which generally serves to reduce tape noise by randomizing the state of the magnetic domains. Disk drives generally re-use tracks by overwriting them; however, application of track-width fields ranging from DC to random bit patterns are also known to have been used for information erasure.
Bulk degaussers can be used in the erasure of magnetic tape when tape recorders lack such function, in which case a decaying AC field is generally used in order to erase the tape so it can be reused. In the case of a disk drive, bulk degaussing can destroy and thereby protect information, for example, when the write mechanism fails but the read mechanism remains operational. The prior art also extends bulk degaussing to protection of information on tapes. Bulk degaussers erase media faster and more thoroughly than tape recorders and disk drives and relieve the operating stress required for recorders and drives to erase media.
Bulk degaussers generally apply large magnetic fields to media in the form of tape wound on reels in tight spirals approaching concentric cylinders or concentric circular tracks on flat disks stacked in a disk drive. The effectiveness of the intended information erasure depends both on the strength of the magnetic field and its direction or range of directions relative to the information-bearing tracks on the tape and disks. The circular direction of tracks on disks and tape reels is the single most effective direction, and degausser configurations suited to apply that direction to either form can generally apply it to the other form about as well.
The strength of the degaussing field must exceed the variable switching strength of the magnetic domains in the media. The most efficient system is one that utilizes a degaussing field which is the most uniform throughout the entire volume of the media area. This allows for operation of the coils at lower power and thus lower energy usage.
The optimal operation is also obtained when the electrical and magnetic parameters of the coil or coils are properly matched. The relationship between values of inductance (L), capacitance (C), and stray resistance (R), which is generally dominated by construction details of the coil, determines whether or not the resulting circuit discharges with an exponentially decaying sinusoidal current or not. When a coil is split into two windings, the windings should have nearly equal L, C, and R. If the two windings are of nearly equal L and C, then the natural oscillation of the coils occurs at approximately the same frequency, thereby minimizing any corrective variances required by external means. Equal L and C and similar field intensity per stored energy squared (H/W2) characteristics of prior art coils also allow charging of capacitances through simple circuitry to equal potential (V) which simplifies design of external switching circuitry.
Practical windings necessarily exhibit some stray resistance (R). In order for energy in two resonant circuits having equal L and C to decay at approximately equal rates, R must be approximately equal. Windings of wire of the same gauge may have equal R when the number of turns and thus length of the coil are the same. As long as some inevitable electrical resistance remains well under a value critical to the values of capacitance and inductance, the natural transient response of such under damped second order series LCR circuitry is an exponentially decaying sinusoid where the values of inductance and resistance alone dictate the rate of decay and the values of capacitance and inductance approximately dictate the frequency of the sinusoidal oscillations; the influence of the resistance on that frequency being limited by its value well under the critical damping value.
In the prior art of bulk degaussing, essentially circular data tracks on magnetic recording tapes and disks systems commonly used phased currents in orthogonal windings where the media remains stationary and the direction of a magnetic field equal to or greater than media size rotates throughout it. Generally, short coils as depicted in U.S. Pat. No. 4,617,603 can generate magnetic fields of essentially uniform strength in the longitudinal direction of the media tracks but not everywhere in radial directions transverse to disk tracks and perpendicular to tape tracks. Short coils are more easily manufactured as they are easy to wind and match in electrical and magnetic properties; however, they cannot provide benefits of radial exposure over media sizes that approach the maximum volume capacity within the magnetic field area.
U.S. Pat. No. 5,270,899 discloses a bulk degaussing system including an erasing head to which is applied a ringing signal to generate a magnetic field for erasing information on a tape media. The system does not provide for specific matching of coils or for varying inductance for erasure. Further, the degaussing system does not provide coverage over the tape media. Prior art depicted in that patent uses an artistic technique of partial hatch on a single coil which may appear as a multiply segmented coil.
Windings that attempt to encompass the media volume more completely, as depicted in U.S. Pat. No. 5,969,933, differ greatly from each other in geometry. Therefore they can be more difficult to match in the desired magnetic and electrical characteristics such as field generation and inductance. Matching various parameters increases energy efficiency and uniform magnetic fields throughout the media area which simplifies design and manufacturing inventory.
Problems can arise when adapting the prior art to be used with a new form of magnetic media. For example, if a longer length is desired, one may be enticed to increase the winding length. However, adding turns to achieve a particular winding length can increase L much beyond the preferred value as L increases in proportion to the square of turns. Also adding turns may increase the magnetic field strength at the center of the coil to unnecessary levels. As a rule of thumb, field intensity generated by a cylindrical winding falls by 50 percent from the center of the winding to the ends of the winding. In order to assure beneficial radial field strengths at the ends of the media area, it is necessary to select operating parameters which generate high operating fields in the center region. These higher parameters increase power requirements and complicate associated switching circuitry.
Helmholz coils have been used to generate magnetic fields of increased uniformity over a specified volume. One species of Helmholz coils employs two separate identically wound coils wired in series where the mean radius is equal to the mean coil spacing. Helmholz coils are generally uniquely designed for a particular experimental or industrial purpose. The unique nature of each Helmholz coil increases the design and manufacturing costs and makes them unfeasible for practical degaussing of magnetic media.